A purging method and a purge unit for a gas turbine combustor capable of effectively preventing deviation of an exhaust gas environmental regulation value from a regulation range in the case of using a dual-fuel type gas turbine combustor. The gas turbine combustor is provided with nozzles. Which communicate with an oil fuel line where oil fuel flows and a gas fuel line where gas fuel flows and are capable of switching injection fuel between the oil fuel and the gas fuel. A purging method for the gas turbine includes a first purge step of purging the oil fuel line by at least water immediately after the injection fuel is switched to the gas fuel from the oil fuel and a second purge step of purging the oil fuel line by at least water immediately before the injection fuel is switched to the oil fuel from the gas fuel.